According to Virtualization.info, Citrix has just opened the beta program for the next version of XenServer, code-named Project George, which will be entirely free. Citrix says that the beta will be open until the end of this month.

The most interesting capabilities:

Active Directory integration. Specify the AD domain to use for authentication by the pool and use your AD credentials to connect to the pool via XenCenter and ssh. You control which AD users/groups are allowed access.